Digital oscilloscopes fall into two general categories. Single shot oscilloscopes begin sampling an event in real time when a trigger condition is satisfied. The limitations on their sampling speed are determined by the speed of the analog-to-digital converter, and the length of time over which an event may be sampled is limited by the size of the acquisition memory that receives the output from the converter. Random interleave or equivalent time sampling oscilloscopes rely on sampling a recurring event at different points in the event repeatedly over time. A single composite representation of the event is then compiled from each of the samples. In order to prevent aliasing, the time between detection of the trigger condition and the sampling is randomized between samples.
Digital oscilloscopes are seen as an improvement over the earlier analog-type devices in certain respects. In the typical analog oscilloscope, the trace of the electron beam across the phosphorous plane of the cathode ray tube is controlled by the amplitude, e.g., voltage, and time characteristics of the signal being monitored. The vertical displacement of the beam is a function of the instantaneous amplitude, and the beam advances horizontally across the tube at a steady rate in time. Since the signal is not sampled and held in a digital memory, data manipulation and direct comparison to subsequent acquisitions are generally not possible. Statistical analysis is difficult.
Analog oscilloscopes have a number of recognized advantages, however. Higher frequency signals may be monitored since any speed limitations are placed, not by the speed of the analog-to-digital converter, but by the bandwidth of the oscilloscopes' electronics. Further, analog oscilloscopes can monitor the signal continuously or almost continuously. Digital oscilloscopes must capture a signal across the time period allowed by the size of the acquisition memory, then process, and display the signal's representation. The speed at which the digital oscilloscope may make new acquisitions is thus limited by the time it requires to process the previous acquisition.